This laboratory is pursuing an understanding of the mechanisms by which microorganisms activate, modulate, or subvert immune systems. We postulate these events are during the developmental and regulatory processes of regulatory T-cells for forming cells and cytotoxic T-cells. Macrophages and natural cytotoxic (NK) cells may also be effected through modulatory effects upon their precursors of their regulatory cells. We have exploited various bacterial toxins as natural probes to facilitate an experimental delineation of mechanisms in this respect. Using an in vitro modular immune system ('cell complementation system' involving murine spleen immunocytes) which allows us to rearrange the order and relative numbers of toxin treated/or untreated cells when recombined in the immune system. Further progress was made by constructing and cloning perpetual cell lines possessing the functions of many of the native regulatory immunocytes. These phenocopies of regulatory cells are used as targets for the bacterial toxins and subsequently tested for altered function. Because these clones may be stored and revived at will cryogenically, they promote continuity of on-going experiments in using homogenous cells at representative stages of development. We have selected suppressor( - ) clones from one of our suppressor precursor clones (NBP2C2, a CD4+/CD8+ clone), in the presence of SPE. Whereas, clones selected similarly using Et or LPS resulted in subclones retaining the parental phenotype. Cell-free supernatants of Et treated, NFR/N or nude mouse splenocytes supported an 80-90% recovery of suppressive activity by the SPEI selected subclone. Macrophage-depletion negated this activity of supernatants. Selected clones were observed not to differ from their parent in expression of MHC haplotype and expression of an epitope of the TCR-agn. Suppression of the expression of IL-2R in the parent clone by SPE and variously to smaller magnitudes by TSST-1, Pt, and SE-C. The SPE selected subclone expressed markedly less CD8 than its parent. These results have been confirmed and are proposed to largely explain the contra-suppressive activity of SPE and similar bacterial toxins.